This invention relates to gas discharge devices in general and more particularly to a novel apparatus for speeding up the process of starting the generation of gas discharge in a ring laser gyroscope which permits the generation of laser beams therein.
Ring laser gyroscopes are commonly used as angular rate sensors. An integral part of the ring laser gyro is the laser beam source of generator. One form of a laser beam generator comprises a gas discharge device in combination with a plurality of mirrors which define a closed path. The path is usually triangular or rectangular, but a pentagonal path etc. could also be used.
Present day ring laser gyros employ a gas discharge device using an He-Ne gas which is excited by an electric current passing therethrough ionizing the gas creating a plasma. As is well understood by those skilled in the art, the ionized gas produces a population inversion which results in the emission of photons, and in the case of He-Ne, a visible light is generated which is indicative of the plasma. If the gas discharge device is properly positioned with respect to the plurality of mirrors, the excited discharge tube will result in two counterpropagating laser beams to travel in opposite directions along an optical closed-loop path defined by the mirrors.
In some embodiments of ring laser gyros, a unitary body provides the gas discharge device including the optical closed-loop path. Such a system is shown in U.S. Pat. No. 3,390,606 by Podgorski, which is assigned to the same assignee as the present invention. There shown, a unitary block forms an optical cavity. A selected lasing gas is used to fill the optical cavity. Mirrors are positioned around the optical cavity at appropriate locations such that counterpropagating beams are reflected so as to travel in opposite directions along the optical cavity. A gas discharge is created in the gas filled optical cavity by means of an electrical current flowing in the gas between at least one anode and at least one cathode which are both in communication with the gas filled optical cavity.
It should be noted that prior art ring laser gyro systems are usually provided with a pair of electrical currents which flow in opposite directions. Each of the electrical currents create plasma in the gas. The current is established by an applied electrical potential, of sufficient magnitude, between one cathode and one anode. As a consequence of the electrical current through the gas, the current affects gas molecule flow. Since the electrical currents usually flow in at least a portion of the path traversed by the laser beams, the gas flow caused by the electrical current results in a bias or error term in the gyro output. Accordingly, in the practice of ring laser gyros, a pair of electrical currents are usually generated in order to balance the gas flow effects caused by the individual currents. In ring laser systems of the prior art, a pair of electrical currents can be provided by a single cathode and a pair of anodes symmetrically placed relative to the closed-loop path of the laser beams so that gas flow effects caused by one of the electrical currents is balanced by gas flow effects caused by the other one of the electrical currents.
Ring laser gyro systems similar to that just described have two inherent characteristics which hinder the ability to make the device lase. First, the unitary body forming the optical cavity is usually of a very high dielectric index resulting in the existence of stray capacitances between the electrodes and other parts of the structure. These stray capacitances result in electrical charge buildup on or in the cavity creating an electric potential which must be overcome by a start up potential applied between each cathode and anode pair of electrodes to initiate electrical current between the cathode and anode so as to ionize the gas and create the plasma. Further, in some systems, particularly triangular ring lasers, the electrodes are not in a straight line relationship with each other through the cavities, the current through the gas between the cathode and anode must follow at least a pair of connected cavity line segments forming the closed-loop path. As a result, the start up potential required to initiate the electrical currents between each cathode and anode pair is very large and much greater than that required to initiate a current through a straight tube laser. Thus, in the prior art, initiation of electrical currents between the cathode and anode to ionize the gas and subsequently cause the generation of laser beams is unreliable, usually slow to start, and requires a very large start up electrical potential between each cathode and anode pair.